LUNAR webinar, 8th March 2011

Wendy Peters, Naval Research Laboratory

Radio Recombination Lines at Decameter Wavelengths: Prospects for
the Future



Radio Recombination Lines (RRLs) are spectral features seen at a wide range of radio frequencies; at the lowest frequencies (< 150 MHz), they appear as absorption features. The Rydberg atoms which produce the lines are large and fragile, and thus the spectral lines are a good probe of the environments in which the atoms exist, providing information on temperature, density, and pressure. Low frequency RRLs are thought to arise largely in diffuse, low-density cold clouds, but their distribution throughout the Galaxy is poorly known. Their optical depths are an order of magnitude larger than that expected for the HI 21cm lines from next-generation "21-cm cosmology" studies of the Epoch of Reionization and earlier. Thus they represent a potential frequency-dependent foreground to these experiments. I will summarize what is currently known about these lines at decametre wavelengths and discuss the possibility of studying them further with new and planned low frequency instruments such as the first station of the Long Wavelength Array.